1. Field of the Invention
This invention relates to an apparatus for liquid-jet recording by jetting liquid droplets utilizing energy generated by a heat-generating means.
2. Description of the Prior Art
FIG. 1(a) is a cross-sectional plan view showing one example of the conventional liquid-jet recording head, and FIG. 1(b) is a cross-sectional view along the line A--A of FIG. 1(a), where a heat-generating means composed of an electro-thermal transducing parts 2 (as will be hereinafter referred to as "heat-generating part") and an electroconductive parts 3 is formed on a substrate 1, and a protective film (not shown on the drawings) is formed thereon. Each of the heat-generating parts 2 is partitioned by grooved plates 4 to form a liquid passage 5 having a thermal action chamber in which the heat energy generated by said heat-generating means acts on a liquid, and a liquid supply chamber 6. A discharge outlet 7 is provided at one end of the liquid passage 5, and the liquid is jetted from the discharge outlet The liquid to be jetted is supplied through a liquid supply pipe 8 provided at the opposite side of the discharge outlet 7 across the heat-generating means to fill the liquid supply chamber 6 and the liquid passage 5.
The liquid can be jetted from the discharge outlet 7 by the heat generated at the heat-generating parts 2. The heat is generated by applying a predetermined pulse voltage to the electroconductive parts 3 connected with the heat-generating parts 2. When the voltage is applied thereto, the liquid near the heat-generating parts 2 undergoes rapid state changes accompanied by bubble formation by the generated heat energy, and the bubbles rapidly grow within the liquid passage 5. The liquid on the side of discharge outlet 7 is pushed out of the discharge outlet 7 rapidly by the generated pressure to form sputtered liquid droplets. The sputtered liquid droplets are deposited onto a recording material to perform recording. When the applied voltage is turned off, the bubbles contract rapidly and vanish.
In such a liquid-jet head, a protective film is generally provided so that the electro-thermal transducing means having the heat-generating parts 2 and the electroconductive parts 3, i.e., a heat-generating means having a resistor and at least one pair of electrodes electrically connected with the resistor as counterposed to the heat-generating part of the resistor may be protected from any contact with the liquid.
FIG. 2 is a cross-sectional view of detail of the heat-generating part 2 of the liquid-jet recording head shown in FIG. 1(b), where a resistor 9 and an electrode 10 are formed on the substrate 1, and the part only of resistor 9 corresponds to the heat-generating part 2 in FIG. 1 and the part of the resistor 9 which is overlapped electrode 10 corresponds to the electroconductive part 3 in FIG. 1. The resistor 9 and electrode 10 as the heat-generating means is protected from a liquid 12 by a protective film 11.
The resistor 9 and electrode 10 have a risk of deterioration, changes in resistance or breaking-down due to chemical reactions such as oxidation reaction, electrolysis, etc., when brought into contact with the liquid 12. Thus, the protective film 11 is provided to prevent such a risk. Protective film 11 functions properly when it is free of defects, and the resistor 9 and electrode 10 are completely separated from the liquid 12, and a long life of the resistor 9 can be ensured.
However, it is actually very difficult to form such an ideal protective film. In the ordinary manufacturing process, fine defects 13 of less than a few microns are inevitably formed on the protective film 11, as shown in FIG. 2. Furthermore, defects 13 are also formed on the protective film 11 due to the thermal stress caused by the heat generation at the heat-generating part 2 of the resistor 9 or impacts, etc. caused by generation and vanishing of bubbles as described above. The presence of defects 13 allows the liquid 12 to contact with the resistor 9 and the electrode 10 to cause electrochemical reaction. The rate of the electrochemical reaction greatly depends on the species of resistor 9 and electrode 10, heat generation temperature of resistor 9, the species of ions in the liquid, etc. Once defects 13 are formed on the heat-generating part 2, the heat-generating part 2 of resistor 9 is damaged and broken down only with about 10.sup.5 -10.sup.6 applications of voltage, and has no practical durability. Practical durability is such that the resistor 9 (particularly, heat-generating part 2) or electrode 10 may not be damaged even after at least about 10.sup.8 applications of pulse voltage.
Thus, the presence of defects 13 on the protective film 11 shortens the life of heat-generating part 2 of resistor 9, and consequently shortens the life of the head, because breakage of only one resistor can terminate the life of the head, even if the head is of full-line multiorifice type. However, it is very difficult to completely remove the defects 13 as already described above. An increase in the thickness of protective film 11 must be avoided for such reasons as a decrease in thermal efficiency, deterioration of heat response to input signals, etc. Thus, in the production of the conventional recording heads, some heads with a short life are unavoidably involved, and the product reliability is considerably reduced.